InGaAlN diode lasers are of interest as light sources for a number of applications; for example, in high-density optical storage, displays, printing and biomedicine. In many devices and systems associated with these applications, laser sources are needed that are capable of providing an output beam with high wavefront quality. Furthermore, the widespread use and commercial success of many of these systems and devices depend on the ability to provide them at low cost. Consequently, high manufacturing yields and low cost are critical requirements for the light sources needed to construct such systems and devices.
Semiconductor diode lasers based on epitaxially grown layers of at least an n-type lower cladding layer, an undoped active layer with quantum wells and barriers, a p-type upper cladding layer, and a highly p-type doped contact layer, have been fabricated from InxGayAlzN, where 0<=x<=1, 0<=y<=1, 0<=z<=1, and x+y+z=1. These lasers are able to emit in a range of wavelengths that span at least from violet to blue-green wavelengths. Lasers of this type have been fabricated and described in the prior art, see for example, S Nakamura, et al., “The Blue Laser Diode: The Complete Story”, Springer-Verlag, 2000, but such lasers face many challenges in meeting the requirements of high manufacturing yield and low cost, high reliability, and high optical quality of the output radiation.
The substrate materials that are currently available for the epitaxial growth of InGaAlN-based laser-active layers lead to unique problems which present substantial obstacles to achieving high manufacturing yield and low cost. For example, available substrates cause unusually high defect densities in the laser-active material layers and, in addition, make it very challenging, if not impossible, to use mechanical cleaving for the formation of laser mirrors due to the mechanical properties of the substrate material. Substrates made of SiC and Sapphire have been used for the fabrication of InGaAlN lasers, but these materials do not permit lattice-matched growth of the InGaAlN layers, and result in very high defect densities, low manufacturing yield and reliability concerns. Recently, freestanding GaN substrates have become available for use in the fabrication of GaN lasers, as described in United States Patent Application Publication No. US 2003/0145783 A1 of Kensaku Motoki, et al, published Aug. 7, 2003. However, even when the highest-quality GaN substrates are used, the laser active layers exhibit a defect density of around 105 cm−2, which is several orders of magnitude higher than for typical commercial semiconductor lasers based on other material systems. Furthermore, the size of these GaN substrates is currently limited to diameters of 2 inches, at most, and the cost is very high. If a low cost is to be achieved, it is important to limit the impact of the defect density on the laser fabrication yield so as significantly to improve yield.
It is known that mirror facets can be formed on diode lasers by etching techniques, as described in U.S. Pat. No. 4,851,368, and in Behfar-Rad, et al, IEEE Journal of Quantum Electronics, volume 28, pages 1227-1231, 1992, the disclosures of which are incorporated herein by reference. However, early work in etching GaN mirror facets did not result in high-quality facets. For example, etched surfaces that were desired to be perpendicular to the substrate turned out at an angle from the vertical, as described in Adesida, et al, Applied Physics Letters, volume 65, pages 889-891, 1994, and the facets were too rough, resulting in poor reflectivity, as described in Stocker, et al, Applied Physics Letters, volume 73, pages 1925-1927, 1998.
Recently, a novel process that allows high quality mirror facets to be formed in a GaN material system has been described in U.S. application Ser. No. 11/455,636, to Behfar et al, filed Jun. 20, 2006, and assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference. As described in that application, it is very difficult to form multiple lasers of short cavity length on a wafer through the use of conventional cleaving techniques because of the mechanical handling that is involved in the cleaving operation. In addition, cleaving results in the simultaneous formation of mirror facets and the singulation of the wafer substrate into separate laser chips. Successful formation of cleaved facets is particularly difficult for InGaAlN based lasers grown on GaN substrates, since the cleaving of GaN crystals is more challenging than cleaving of the GaAs and InP substrates previously used for the mass-produced diode lasers utilized for CD, DVD and telecommunications.
On the other hand, use of the etching process described in application Ser. No. 11/455,636 for the formation of laser facets permits optimization of the facet formation independently of the subsequent device singulation. In this process, lasers are fabricated on a wafer in much the same way that integrated circuit chips are fabricated on silicon, so that the chips are formed in full-wafer form. The laser mirrors are etched on the wafer using etched facet technology (EFT), and the electrical contacts are fabricated on the lasers. The lasers are tested on the wafer, and thereafter the wafer is singulated to separate the lasers for packaging. Scanning Electron Microscope images of etched AlGaInN-based facets show that a high degree of verticality and smoothness can be achieved using the EFT process, which also allows lasers and integrated devices to be fabricated for a variety of applications having wavelength requirements accessible with AlGaInN-based materials.
The foregoing process for fabricating lasers can be summarized as comprising the steps of lithographically defining a multiplicity of waveguide devices on a wafer having an AlGaInN-based structure and etching through the resulting mask to fabricate a multiplicity of laser waveguide cavities on the wafer. Another lithographic step followed by etching is used to form laser facets, or mirrors, on the ends of the waveguides while they are still on the wafer. Thereafter, electrical contacts are formed on the laser cavities, the individual lasers are tested on the wafer, and the wafer is singulated to separate the lasers for packaging. This method of etching the facets includes using a high temperature stable mask on a p-doped cap layer of the AlGaInN-based laser waveguide structures on the wafer to define the locations of the facets, with the mask maintaining the conductivity of the cap layer, and then etching the facets in the laser structure through the mask using a temperature over 500° C. and an ion beam voltage in excess of 500V in CAIBE.
Selectivity between the etching of the semiconductor and the masking material is very important in obtaining straight surfaces for use in photonics. High selectivity between the mask and the GaN based substrate is obtained by performing CAIBE at high temperatures. Large ion beam voltages in CAIBE were also found to enhance the selectivity. The mask materials were chosen to withstand the high temperature etching, but also to prevent damage to the p-contact of the GaN-based structure.
Particularly in the case of InGaAlN lasers, etching of the laser mirrors can offer a number of important advantages for improving yield and reducing cost. For example:                (a) The laser cavity waveguide dimensions can be different from the chip length dimension and can be optimized to maximize the laser fabrication yield. By fabricating a waveguide of limited length, the probability of a material defect occurring in the laser active region is reduced and the fabrication yield is increased.        (b) Redundant lasers can be fabricated on one semiconductor chip to produce yield and reliability improvements.        (c) Surface emitting lasers with the laser cavity oriented horizontally in the wafer plane can be fabricated by etching a 45° surface to direct the radiation upward out of the wafer plane.        (d) Laser facet coatings for desirable reflectivity modifications can be applied at the full wafer level prior to device separation.        (e) Laser testing can be carried out economically at the full-wafer level.        (f) Additional components such as photodiodes, lenses and gratings can be monolithically integrated with the lasers.        
The yield and cost of today's mass-produced diode lasers based on GaAs and InP substrates are not impacted by the substrate quality and cost. Substrates for these laser devices typically have defect densities of about 102 cm−2 and are available in wafers of larger sizes of up to 6 inches in diameter at a cost that is several orders of magnitude lower than that of GaN substrates. Both GaAs and InP have a zinc blend crystal structure that facilitates the use of cleaving for both the formation of the laser end mirrors and the chip singulation, and cleaving is the primary method used in volume production of these semiconductor lasers. In addition, in diode laser applications in areas such as telecommunications optical imaging is not a primary concern and the requirements on optical beam quality are more relaxed.
What is needed in order to produce InGaAlN lasers with high yield, low cost, high reliability and good wavefront quality is a device design that minimizes the occurrence of substrate-induced defects in and near the laser-active region, and provides an undistorted optical beam and a method for fabricating such a laser device.